1. Field of the Invention
The present invention relates to an optical transmission system using fibers, and more particularly to an optical transmission system using in-line amplifiers.
2. Description of the Related Art
An optical transmission system is now being developed with increased capacity and an extended span of transmission. An increase of a bit rate and wavelength division multiplexing system are now being discussed so as to increase capacity. In the meantime, an optical amplifier has been introduced so as to extend the span of transmission. The types of optical amplifiers include a post-amplifier (for strengthening output of transmission power), a pre-amplifier (for increasing the sensitivity of reception power), and a repeater (in-line amplifier). The optical amplifier is currently under development at a production level. The introduction of the optical amplifier allows the difference between optical intensities at transmission and reception to be extended, and an allowable loss in the fiber is increased.
A system configuration using a post-amplifier and a pre-amplifier has been put into practical use. Additionally, the in-line amplifier is under development in order to extend the reproduction relay interval. Here, the in-line amplifier is a repeater which amplifies an optical signal unchanged without converting it into an electric signal, and transmits the amplified signal.
The system using in-line amplifiers, however, poses a new problem where amplified spontaneous emission lights, from plurality of amplifiers, due to the connection of the plurality of amplifiers, are accumulated, and the S/N ratio is lowered. The lowering of the S/N ratio leads to the degradation of a minimum reception power of a receiver. To obtain a predetermined system gain in consideration of this degradation, transmission power output must be strong thereby a lower limit value of the transmission power is determined. Furthermore, if the transmission power output is stronger than a threshold (+8 dBm for a dispersion shifted fiber, and 10 dBm or more for a single mode fiber, although it depends on the length of a transmission path or a wavelength), the waveform is significantly degraded due to the non-linear effect of a fiber. One type of wavelength degradation is an optical Kerr effect (refractive index changes depending on an optical intensity). This is a phenomenon where a frequency (wavelength) shift occurs at the rising and falling edges of an optical signal pulse (SPM: Self-Phase Modulation). Even if the width of an optical wavelength is narrow before transmission, the width of the wavelength increases, and at the same time, a reception waveform significantly changes due to the influence of fiber dispersion. The upper limit of optical transmission power is determined in consideration of such an influence.
Fiber dispersion means that the speed of light propagating on a fiber depends on its wavelength. An optical pulse having a certain wavelength width is widened or compressed after fiber propagation. This effect is referred to as fiber chromatic dispersion. Accordingly, a reception waveform in an optical transmission system after fiber propagation varies depending on the chromatic dispersion, and a transmission error will occur depending on the degree of dispersion. Therefore, the fiber dispersion imposes a restriction on the transmission distance.
With a system using an in-line amplifier which amplifies an optical signal without conversion, such non-linear effect and dispersion are accumulated while the optical signal travels. Accordingly, it becomes quite impossible to properly receive the optical signal on a receiving side unless suitable compensation is made.
In the meantime, a system implemented by combining blue chirping on a transmitting side and dispersion compensation in repeaters and a receiver was conventionally proposed.
FIG. 1 is a schematic diagram showing a combination of conventional pre-chirping and dispersion compensators.
In this figure, a transmitter 1000 and a receiver 1010 are connected by transmission paths 1003, 1006 and 1009, and repeaters 1004 and 1007. The transmitter 1000 is composed of an E/O 1001, for converting an electric signal into an optical signal, and a post-amplifier 1002. The transmitter 1000 performs blue-chirping on the optical signal, and transmits the signal. The transmitted optical signal travels along the transmission path 1003 and enters the repeater 1004. The repeater 1004 amplifies the optical signal, and performs dispersion compensation using the dispersion compensator 1005. The amount of dispersion compensation is a constant value. The optical signal, which is further amplified and dispersion-compensated, passes along the transmission path 1006 and enters the repeater 1007. The repeater 1007 also amplifies the signal, performs dispersion compensation and transmits the signal on the transmission path 1009. The optical signal passes through repeaters whose number is predetermined, until it reaches the receiver 1010. The receiver 1010 amplifies the received optical signal using a pre-amplifier, performs dispersion compensation using a dispersion-compensator 1012, inputs the signal to an O/E 1013 in order to convert the optical signal into an electric signal, and extracts necessary data.
That is, the conventional combination is implemented by combining blue chirping (especially, chirping parameter=−1) as the pre-chirping, and compensation by the dispersion-compensators arranged in in-line amplifiers and the receiver (between the pre-amplifier and the O/E). If the blue-chirping is performed in a fiber of +dispersion, the output pulse is compressed due to the characteristics of the fiber of +dispersion, and the chirping. As a result, a transmission distance is made relatively longer. Especially, in a system which does not use optical amplifiers, an optical signal having the wavelength of 1.5 μm is more effective when it travels along a single mode fiber (1.3 μm zero-dispersion wavelength). Accordingly, dispersion compensation implemented by combining the pre-chirping and the succeeding compensation was considered also to be effective in a system using optical amplifiers. If the amount of dispersion compensation is set in order to keep a residual dispersion value (obtained by subtracting the amount of dispersion compensation from a total amount of dispersion of a transmission fiber) constant, a stable transmission characteristic can be obtained.
However, if output of the transmission power is increased by introducing optical amplifiers according to this method, the influence of the non-linear effect of an optical fiber remarkably appears. The influence of the non-linear effect is equivalent to the characteristic of blue chirping. The pulse width of the transmission waveform is narrowed due to the influence of the pre-chirping at the transmitter and the non-linear effect of the optical fiber. As a result, the influence of the non-linear effect remarkably appears, and the waveform is significantly changed for the dispersion.
The problems posed by the method for performing blue-chirping at the time of transmission are listed below.    1) Output of transmission power cannot be increased.    2) Dispersion-compensation on a transmitting side is ineffective.    3) The dispersion-compensation is performed in in-line amplifiers and on a receiving side due to the ineffectivity on the transmitting side in consideration with 2). Accordingly, the losses of dispersion-compensators become larger, and the tolerance of the losses becomes difficult as transmission distance is extended. Lowering the level of an optical input to the O/E leads to degradation of reception sensitivity, and this imposes a limitation. Furthermore, optical input power may sometimes have an upper limit depending on the dispersion-compensator to be used.    4) The tolerance of the amount of dispersion-compensation which can ensure the transmission characteristic is small.    5) The number of types of different devices increases when the dispersion-compensators are prepared according to a transmission distance due to the small tolerance as a result of 4).